ESA’s EnVision mission to Venus will perform optical, spectral and radar mapping of Earth’s sister planet. But before getting to work, the van-sized spacecraft must “aerobrake” – lowering its orbit with thousands of passes through the planet’s hot, thick atmosphere for up to two years. A unique ESA facility is currently testing candidate materials for spacecraft to verify that they can safely withstand this harsh process of atmospheric surfing.
“EnVision as it is currently designed cannot do without this long aerobraking phase,” explains Thomas Voirin, head of the EnVision study at ESA.
“The spacecraft will be injected into Venus orbit at a very high altitude, about 250,000 km, and then we have to descend into a polar orbit at 500 km altitude for science operations. Flying on an Ariane 62, we can’t afford all the extra propellant it would take to lower our orbit. Instead, we’ll be slowing down with repeated passes through Venus’ upper atmosphere, coming as low as 130 km from the surface.
EnVision’s predecessor spacecraft, Venus Express, performed experimental aerobraking during the final months of its mission in 2014, gathering valuable data on the technique. Aerobraking was first used in 2017 by ESA’s ExoMars Trace Gas Orbiter (TGO) to lower its orbit around the Red Planet over an 11-month period.
Thomas notes: “The aerobraking around Venus is going to be much more difficult than for TGO. For starters, Venus’ gravity is about 10 times that of Mars. This means that velocities about twice as high as for TGO will be experienced by the spacecraft as it passes through the atmosphere – and the heat is generated in the form of a cube of velocity. As a result, EnVision must target a lower aerobraking rpm, resulting in a twice longer aerobraking phase.
“On top of that, we’re also going to be much closer to the Sun, experiencing about twice the solar intensity of Earth’s, with the thick white clouds in the atmosphere reflecting a lot of sunlight directly into space, which must also be taken into account. Then, on top of all this, we realized that we had to reckon with another factor on the thousands of orbits that we are considering, previously only experienced in low Earth orbit: highly erosive atomic oxygen.
It is a phenomenon that remained unknown during the first decades of the space age. It wasn’t until the first space shuttle flights returned from low orbit in the early 1980s that engineers received a shock: the spacecraft’s thermal blankets had been badly eroded.
The culprit turned out to be highly reactive atomic oxygen – individual oxygen atoms at the fringes of the atmosphere, the result of standard oxygen molecules of the type found just above the ground being shattered by powerful radiation ultraviolet of the Sun. Today, all missions below about 1,000 km must be designed to withstand atomic oxygen, such as the Copernicus Earth observation Sentinels in Europe or any hardware built for the International Space Station.
Spectral observations by ancient Venus orbiters of the airglow above the planet confirm that atomic oxygen is also prevalent at the top of Venus’ atmosphere, which is more than 90 times thicker than the planet. Earth’s ambient air.
Thomas says: “The concentration is quite high, with one pass it doesn’t matter that much but over thousands of times it starts to build up and ends in a fluence level of atomic oxygen which we must take into account, equivalent to what we experience in low Earth orbit, but at higher temperatures.
The EnVision team turned to a unique European facility specially built by ESA to simulate atomic oxygen in orbit. The low Earth orbit facility, LEOX, is part of the Electrical Materials and Components Laboratorybased at ESA’s ESTEC Technical Center in the Netherlands.
Adrian Tighe, Materials Engineer at ESA, explains: “LEOX generates atomic oxygen at energy levels equivalent to orbital velocity. Purified molecular oxygen is injected into a vacuum chamber with a pulsed laser beam focused on it. This transforms the oxygen into a hot plasma whose rapid expansion is channeled along a conical nozzle. It then dissociates to form a highly energetic beam of atomic oxygen.
“To operate reliably, the laser timing must remain accurate to the millisecond scale and directed with precision measured to the thousandth of a millimeter, throughout the four months of this current test campaign.
“This is not the first time the facility has been used to simulate an extraterrestrial orbital environment – we have already performed atomic oxygen tests on candidate solar panel materials for ESA’s Juice mission, as the Telescopic observations suggest that atomic oxygen will be found in the atmospheres of Europa and Ganymede, however for EnVision the high temperature during aerobraking poses an additional challenge, so the facility has been adapted to simulate this environment more extreme Venusian.
A range of materials and coatings from different parts of the EnVision spacecraft, including multi-layered insulation, antenna parts and star tracking elements, are placed in a plate to be exposed to the purple glowing LEOX beam. At the same time, this plate is heated to mimic the expected heat flux, up to 350°C.
Thomas adds: “We want to verify that these parts are resistant to erosion, and also retain their optical properties, i.e. that they do not degrade or darken, which could have repercussions on their thermal behavior, because we have scientific instruments that must maintain a defined temperature. We must also avoid flaking or outgassing, which leads to contamination.
This current test campaign is part of a larger panel examining EnVision aerobraking, including the use of a Venus climate database developed from the results of previous missions to estimate the local variability of the atmosphere of the planet in order to define safety margins for the spacecraft.
The results of this test campaign are expected by the end of this year.
EnVision is an ESA-led mission in partnership with NASA, providing its synthetic aperture radar instrument, VenSAR and Deep Space Network for mission critical phases. EnVision will use a range of instruments to make holistic observations of Venus from its inner core to the upper atmosphere to better understand how Earth’s nearest neighbor in the solar system has evolved so differently.
EnVision has been selected by ESA’s Science Program Committee as the fifth medium-class mission in the Agency’s Cosmic Vision plan, aiming for launch in the early 2030s.